Provide a UUID in edittemplate templates.

I want to make GUIDs for my RSS feeds that don't change when I move
pages around.  To that end, I've used UUID::Tiny to generate a
version 4 (random) UUID that is presented in a `uuid` variable in
the template.

At that point, you can do something like this:

    [[!meta guid="urn:uuid:<TMPL_VAR uuid>"]]